1. Field of the Invention
The invention relates to a circuit for recovering the carrier of an amplitude- and- phase-modulated ASK-PSK input signal (in particular QAM signal), comprising an oscillator having a frequency control for producing said carrier, a quadrature detector for supplying signals representing the amplitude of said input signal projected onto two quadrature components P and Q of the carrier signal produced by said oscillator and for thus determining the modulation states of said input signal, and correction means for forming a correction signal for said frequency control of said oscillator on the basis of output signals from a circuit which determines zones in the plane of said quadrature components P and Q.
2. Description of the Prior Art
A circuit of this kind has been described in U.S. Pat. No. 3,334,312 corresponding to French Patent Application No. 2,466,145 filed on Aug. 11, 1980. In this known circuit, the correction signal is produced with all the modulation states of the input signal considered as having the same importance which ever zone they may be in. If the input signal is transmitted by a channel subject to the phenomenon of selective fading, the position of the states relative to the components in quadrature is no longer that of the ideal points but depends on the succession of states transmitted. The uncertainty of these points is more important accordingly as the phenomenon of selective fading is more pronounced. It may even happen that the domains of uncertainty become adjacent. In that case the known circuit functions badly because the states for which the signal amplitude is weak are noise generators and entail significant errors for the correction signal.